The need for purification, disinfection, and sterilization of water to produce water safe for human contact and consumption is of growing importance in recent times. More and more of the nation's water supply has become polluted and unsafe for human use.
Virtually all metropolitan areas employ various water treatment processes to eliminate suspended solids, unpleasant tastes and odors, and harmful bacteria from their water supply. It would, of course, be more desirable to use water purified by natural means such as sunlight and aeration for surface water and percolation and infiltration through soil for ground water, but such pure sources of water are increasingly scarce. Recently, even some rural areas must purify water before use by humans and animals.
Virtually all surface water employed for human contact and consumption must be disinfected before use. Filtration, coagulation and similar processes are employed to remove solid matter from water, but disinfection is required to remove harmful microorganisms, bacteria and the like. Examples of such bacteria are organisms of the coliform group and fecal streptococci.
The most common water disinfection method employed at the present time is chlorination. The degree of contamination of the water determines the amount of chlorine necessary to disinfect the water, and many municipal water supplies must be treated with so much chlorine that the consumer can actually smell and taste the chlorine in the water. Such a concentration of chlorine may be required to disinfect the water but it is unpleasant to the consumer.
Silver ions have long been known to be a good antiseptic and disinfecting agent. Very minute quantities of silver exercise great bactericidal power, but prior attempts to employ silver in commercial water disinfecting processes have met with limited success. One reason for such limited success is the necessity to maintain the silver concentration in the water at an acceptable and safe level for human or aninal consumption. Various authorities differ on a safe level of silver concentration, but it is noted that a silver concentration of 0.2 parts per million (ppm) (0.200 mg/l) can be of no physiological significance. The Public Health Service has viewed a concentration of silver in excess of 0.050 mg/l (milligrams per liter) as grounds for rejection of the supply of silver-treated water.
Silver ions, in concentrations sufficient to kill bacteria, do not affect the color, taste or smell of water. But, at the maximum safe concentration levels or below, many types of bacteria are not killed instantly, so some residence time is required to achieve sterilization. Silver ions may be introduced into water by several conventional means such as chemical ion exchange or by electrolysis.
A considerable demand exists for a practical method and apparatus for treating water from small, single dwelling wells such as the type of well found on a farm. Since long residence times for disinfecting water are not practicable for water coming from a well as compared to a municipal treatment facility, some practical method and apparatus for raising the level of silver concentration above the safe, maximum levels, so that disinfection can proceed at a faster rate, and then lowering the concentration to within safe levels so as to produce potable water, would find great use in conjunction with unsafe farm wells.
Chlorination is also a typical method used to treat and disinfect swimming pool water to kill harmful bacteria. Chlorine is frequently objectionable because it has a strong odor and it causes the unpleasant sensation of burning eyes that swimmers frequently feel. A need therefore exists for a method and apparatus which will kill harmful bacteria in swimming pool water and yet will neither create objectionable odors nor be irritating to the swimmer.